It is known that after a period of time heterogeneous, supported catalysts used in the decarbonylation of aldehydes at an elevated temperature become deactivated, for example by carbonaceous deposit (often called coke) on the catalyst. Such deactivation is shown by a lower conversion, lower selectivity and/or lower yield of the desired decarbonylated compound. An example of such decarbonylation is the decarbonylation of furfural (2-formylfuran) to furan, as shown below. Furan is an important intermediate in the production of tetrahydrofuran (THF) and 1,4-butanediol (BDO).

Deactivated decarbonylation catalysts need to be regenerated again. It is known to regenerate by burning off coke from such deactivated catalysts by using a gas stream comprising oxygen, such as an air stream, at an elevated temperature.
For example, WO2010080290 discloses a process for the decarbonylation of specified aldehydes, including furfural, using a Pd/alumina catalyst that has been promoted with an alkali carbonate while heating. Further, WO2010080290 discloses regenerating said catalyst by feeding air, or by feeding a mixture of air and steam or nitrogen, at a temperature of 300-500° C.
In Example 2 of WO2010080290, furfural was decarbonylated using hydrogen and a Pd/alumina catalyst treated with cesium carbonate. The initial reaction temperature was 270° C. After the conversion of furfural dropped, the reaction temperature was raised to 280° C. and then to 290° C. in order to increase the furfural conversion. Such temperature increase during the decarbonylation of furfural, while furfural is still being fed, may initially result in a somewhat higher conversion but in the long term it will disadvantageously result in an increased catalyst deactivation rate. Table 2 of WO2010080290 shows that the final furfural conversion was only 39.2%. Therefore, such treatment does not regenerate the catalyst.
In Example 3 of WO2010080290, an air feed and a vaporized water (steam) feed were used, said water feed containing about 2 vol. % of oxygen, to regenerate the catalyst from Example 2 by burning off the carbon from the catalyst at an elevated temperature (330-350° C.). The reactor was then purged with nitrogen and the air and water feeds were stopped. The regenerated catalyst was then tested again for furfural decarbonylation, by lowering the temperature (to 290° C.) and resuming a hydrogen flow and a furfural flow. In Example 5 of WO2010080290, an air feed and a nitrogen feed were used for regeneration.
Also WO2010071745 discloses regeneration of furfural decarbonlyation catalysts using air.
Oxidative catalyst regeneration using air is a cumbersome treatment that implies multiple operation steps, dedicated equipment to feed the reactor with either pure N2 (for purge) or air (for coke burn-off). Further, this requires an accurate reactor monitoring for avoiding runaway during the coke burn-off. By applying oxidative catalyst regeneration there is also the hazard connected with a possible mixing of H2 that may be used for the decarbonylation reaction and O2 needed for the regeneration. A further drawback of oxidative catalyst regeneration is that this cannot be applied to catalysts which comprise carbon as a support, because the carbon support would also be burnt under such oxidative conditions.
It is an object of the present invention to provide a process for regenerating a heterogeneous, supported catalyst used in the decarbonylation of an aldehyde, which process does not have the above drawbacks.